Set the correct media type for login response and parse the Accept header

[marmoure]

This PR adds parsing for Accept header and introduces local:accept-json and local:preferred-media-type, it build on top of #800 to fix #253.
These functions allow more robust and future proof way for getting the appropriate header either the preferred one or a specific one.

Also when trying to set the correct headers for the login response using

util:declare-option("output:method", "json"),
util:declare-option("output:medita-type", "application/json"),

I encountered a bug in the RESTServer in writeResultJSON which ignored the output:medita-type option, I added evolvedbinary/elemental#212 for elemental and the same should be done for eXist-db.

In the mean time

response:set-header("Content-Type", "application/json; charset=UTF-8"),

is being used as workaround until this bug is resolved.

[duncdrum]

Thanks for the PR @marmoure. The Xqdoc is not well-formed.

We don't normally link elemental issues. Feel free to keep it in alongside an exist issue. But both should be properly formatted.

Please add a test. Did you test this against exist? Which version?

[marmoure]

Thanks for the @duncdrum I fixed the xqdoc issue, this was tested on eXist-db 6.4.1.